how much to offer

i just found today an 90 mustang convertable 5.0 auto down the road and the guy said its his friends and he might sale it. the only thing wrong with it is that the trany is shot but they have anouther one ready to go in the all the power stuff works but the drivers side window. i know i can put the motor in for the window and drop the trany and put it in myself so those are not that big a deal. The car has no major dents in it and the paint is good shape but it needs a good cleaning inside and out and the top is really good what do you guys think i should offer the guy for the car i was thinking if they would throw the trany in I would offer maby 1000 or 1500 what do you guys think.